(d) Alkenes

Learning objectives

4.23Alkene functional groupKnow that alkenes contain the functional group >C=C<4.24Alkene general formulaKnow the general formula for alkenes 4.25Unsaturated hydrocarbonsExplain why alkenes are classified as unsaturated hydrocarbons 4.26Alkene structures and isomersUnderstand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers knowledge of cis/trans or E/Z notation is not required 4.27Alkene addition with bromineDescribe the reactions of alkenes with bromine to produce dibromoalkanes 4.28Bromine-water testDescribe how bromine water can be used to distinguish between an alkane and an alkene

Every alkene contains the functional group $>\!\ce{C=C}\!<$ : a carbon–carbon double bond with each carbon also joined to the rest of the molecule.

Look for two adjacent carbon atoms connected by two bond lines. In ethene, $\ce{CH2=CH2}$ , the $\ce{C=C}$ bond is the functional group; in propene, $\ce{CH2=CHCH3}$ , the same group is attached to a $\ce{CH3}$ group.

The functional group is specifically $\ce{C=C}$ , not any double bond and not the whole molecule. A $\ce{C=O}$ bond does not make a compound an alkene.

The homologous series of open-chain alkenes with one carbon–carbon double bond has the general formula $\ce{C_nH_{2n}}$ , where $n$ is the number of carbon atoms and $n\ge 2$ .

$n$	Alkene	Molecular formula
2	ethene	$\ce{C2H4}$
3	propene	$\ce{C3H6}$
4	butene isomers	$\ce{C4H8}$

Substitute the carbon count for $n$ and double it to obtain the hydrogen count. A five-carbon member therefore has formula $\ce{C5H10}$ .

Do not use the alkane formula $\ce{C_nH_{2n+2}}$ . The $\ce{C_nH_{2n}}$ pattern here assumes one double bond and no ring; molecules with several double bonds follow a different hydrogen count.

An alkene is an unsaturated hydrocarbon because it contains only carbon and hydrogen, and it has at least one carbon–carbon double bond.

Word	Structural evidence
hydrocarbon	only carbon and hydrogen atoms are present
unsaturated	a $\ce{C=C}$ bond is present, so the molecule can add atoms across that bond

During an addition reaction, the double bond becomes a single bond and each of its carbon atoms forms a new bond. This capacity to add atoms distinguishes an unsaturated alkene from a saturated alkane.

A compound can contain a $\ce{C=C}$ bond yet fail to be a hydrocarbon if it also contains another element. Both parts of the classification must be justified.

An alkene displayed formula must show every atom and bond, including one $\ce{C=C}$ bond; each carbon must have four bonds in total and each hydrogen one.

Carbon atoms	Unbranched name	Condensed structural formula
2	ethene	$\ce{CH2=CH2}$
3	propene	$\ce{CH2=CHCH3}$
4	but-1-ene	$\ce{CH2=CHCH2CH3}$
4	but-2-ene	$\ce{CH3CH=CHCH3}$

Choose the longest chain containing the double bond. Number from the end that gives the double bond the lowest position, then place that number before “ene”: but-1-ene has its double bond starting at carbon 1; but-2-ene starts at carbon 2.

Structural isomers have the same molecular formula but different connectivity. $\ce{C4H8}$ includes but-1-ene, but-2-ene and branched methylpropene; cyclic alkanes are not alkene answers.

Turning a drawing around does not create a new structural isomer. Cis/trans or E/Z notation is not required, so do not count those labels as extra required names.

Alkenes react with bromine by addition to form dibromoalkanes: the $\ce{C=C}$ double bond becomes a single bond and one bromine atom bonds to each of the two carbon atoms.

\ce{CH2=CH2 + Br2 -> CH2Br-CH2Br}

Ethene forms 1,2-dibromoethane. No atoms are removed: both bromine atoms from $\ce{Br2}$ appear in the one saturated product, while the carbon skeleton is unchanged.

This is addition, not alkane substitution. Ultraviolet radiation is not required, and the product retains no $\ce{C=C}$ bond at the reacted position.

Add bromine water to separate samples and shake: an alkene decolourises the bromine water from orange to colourless, whereas an alkane leaves it orange under these test conditions.

Sample	Observation	Conclusion
alkene	orange bromine water becomes colourless	$\ce{C=C}$ is present and bromine adds across it
alkane	no colour change; bromine water stays orange	no $\ce{C=C}$ is present

Name bromine water, not bromide or bromine alone, and report the colour change—not merely “a reaction occurs”. This comparison is made without ultraviolet radiation; UV would introduce the different alkane substitution reaction.

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—(e) AlcoholsRead

(d) Alkenes

Learning objectives

Locate the alkene functional group

Use the alkene general formula

Explain why alkenes are unsaturated hydrocarbons

Draw and name alkenes up to four carbons

Follow bromine addition across a double bond

Distinguish an alkane from an alkene with bromine water

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